Generally, a prosthesis may be implanted into a bone by forming a cavity into the bone, inserting the prosthesis into the cavity and anchoring the prosthesis to the bone cavity with an adhesive or cement. A common type of bone cement used in retaining a prosthesis in a bone canal comprises a mixture of a liquid monomer component, e.g. methyl methacrylate monomer (MMA), and a polymer powder component, e.g., polymethylmethacrylate (PMMA). The PMMA beads generally contain a small amount of benzoyl peroxide which functions as an initiator when the monomer component and polymer component are mixed. The liquid monomer component includes a small amount of N,N-dimethyl-p-toluidine (DMPT) and hydroquinone (HQ). DMPT promotes cold curing when the liquid monomer component and polymer powder component are mixed. HQ functions as an inhibitor to prevent premature polymerization which may occur under conditions such as heat, light, or chemical reagents. Collectively, the MMA, DMPT, and HQ are referred to as starting materials. The liquid monomer component and powder polymer component are mixed into a paste which is then placed into the bone cavity immediately prior to insertion of the prosthesis stem therein.
In order to maximize the strength of the interface between the prosthesis and the bone cement, the stem of the prosthesis can be precoated in a nonsurgical environment with a polymer coating of PMMA to achieve a chemical bond with the bone cement at the time of surgery. U.S. Pat. No. 4,281,420 discloses the details of how the PMMA coating may be applied to the prosthesis. Such a precoat is typically applied to a metal stem by an electrostatic coating process, with the starting material being a reactor-polymerized PMMA. After insertion of the PMMA precoated prosthesis with the fresh bone cement at the time of implantation, the PMMA coating will become securely bonded to the fresh bone cement as the fresh bone cement polymerizes.
PCT WO 94/20046 discloses a thick polymer or cement mantle which can be preapplied by molding the polymer about the base implant in a nonsurgical or factory environment. This molded polymer layer, like bone cement, is formulated as a self-curing PMMA system. This molded polymer layer relies on a chemical initiator-activator-inhibitor (benzoyl peroxide, DMPT and HQ) system for the self-curing mechanism.
A common method of sterilization is to expose a prosthetic implant to a high energy irradiation process, especially gamma radiation, for a suitable duration. This process can be applied to products through certain types of packaging materials, and as such, is an effective way of providing presterilized packaged products to the customer. However, such radiation can cause the color of the PMMA coating to change from a substantially white or creme or colorless condition to a more yellowed or discolored condition. This is undesirable in that the yellowed or discolored condition is not aesthetically pleasing, and a yellow polymer tends to indicate aging. Accordingly, it is desirable to provide a prosthesis having a polymer coating which remains substantially colorless after radiation sterilization.